Antiallodynic effect of intrathecal gabapentin and its interaction with clonidine in a rat model of postoperative pain

Phosphoproteomic Profiling of Rat's Dorsal Root Ganglia Reveals mTOR as a Potential Target in Bone Cancer Pain and Electro-Acupuncture's Analgesia

Bone cancer pain (BCP) is a clinical refractory mixed pain involving neuropathic and inflammatory pain, with the underlying mechanisms remaining largely unknown. Electro-acupuncture (EA) can partly alleviate BCP according to previous research. We aim to explore the proteins and major pathways involved in BCP and EA treatment through phosphoproteomic profiling. BCP rat model was built by tibial inoculation of MRMT-1 mammary gland carcinoma cells. Mechanical hyperalgesia determined by paw withdrawal thresholds (PWTs) and bone destruction manifested on the radiographs confirmed the success of modeling, which were attenuated by EA treatment. The differentially expressed phosphorylated proteins (DEPs) co-regulated by BCP modeling and EA treatment in rat dorsal root ganglions (DRGs) were analyzed through PEX100 Protein microarray. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that DEPs were significantly enriched in mammalian target of rapamycin (mTOR) signaling pathway. The phosphorylations of mTOR at Ser2448 and Thr2446 were increased in BCP and downregulated by EA. In addition, the phosphorylation of S6K and Akt, markers of the mTOR complex, were also increased in BCP and downregulated by EA. Inhibition of mTOR signaling alleviated the PWTs of BCP rats, while the mTOR agonist impaired the analgesic effect of EA. Thus, our study provided a landscape of protein phosphorylation changes in DRGs of EA-treated BCP rats and revealed that mTOR signaling can be potentially targeted to alleviate BCP by EA treatment.

Mesenchymal stem cell exosomes as a cell-free therapy for nerve injury-induced pain in rats

Nerve injury-induced neuropathic pain is difficult to treat. In this study, we used exosomes derived from human umbilical cord mesenchymal stem cell (UCMSC) as a cell-free therapy for nerve injury-induced pain in rats. Isolated UCMSC exosomes range in size from 30 to 160 nm and contain CD63, HSP60, and CD81 exosome markers. After L5/6 spinal nerve ligation surgery, single intrathecal injection of exosomes reversed nerve ligation-induced mechanical and thermal hypersensitivities of right hindpaw of rats at initial and well-developed pain stages. Moreover, continuous intrathecal infusion of exosomes achieved excellent preventive and reversal effects for nerve ligation-induced pain. In immunofluorescent study, lots of Exo-green-labelled exosomes could be found majorly in the ipsilateral L5 spinal dorsal horn, dorsal root ganglion, and peripheral axons, suggesting the homing ability of UCMSC exosomes. They also appeared in the central terminals or cell bodies of IB4, CGRP, and NF200 sensory neurons. In addition, exosome treatment suppressed nerve ligation-induced upregulation of c-Fos, CNPase, GFAP, and Iba1. All these data suggest that the analgesic effects of exosomes may involve their actions on neuron and glial cells. Exosomes also inhibited the level of TNF-α and IL-1β, while enhanced the level of IL-10, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor in the ipsilateral L5/6 dorsal root ganglion of nerve-ligated rats, indicating anti-inflammatory and proneurotrophic abilities. Protein analysis revealed the content of vascular endothelial growth factor C, angiopoietin-2, and fibroblast growth factor-2 in the exosomes. In summary, intrathecal infusion of exosomes from UCMSCs may be considered as a novel therapeutic approach for nerve injury-induced pain.

Antiepileptic drugs as analgesics/adjuvants in inflammatory pain: current preclinical evidence

Inflammatory pain is the most common type of pain that is treated clinically. The use of currently available treatments (classic analgesics - NSAIDs, paracetamol and opioids) is limited by insufficient efficacy and/or side effects/tolerance development. Antiepileptic drugs (AEDs) are widely used in neuropathic pain treatment, but there is substantial preclinical evidence on their efficacy against inflammatory pain, too. In this review we focus on gabapentinoids (gabapentin and pregabalin) and dibenzazepine AEDs (carbamazepine, oxcarbazepine, and recently introduced eslicarbazepine acetate) and their potential for relieving inflammatory pain. In models of somatic, visceral and trigeminal inflammatory pain, that have a translational value for inflammatory conditions in locomotor system, viscera and head/face, AEDs have demonstrated analgesic activity. This activity was mostly consistent, dependent on the dose and largely independent on the site of inflammation and method of its induction, nociceptive stimuli, species, specific drug used, its route of administration and dosing schedule. AEDs exerted comparable efficacy with classic analgesics. Effective doses of AEDs are lower than toxic doses in animals and, when expressed as equivalent human doses, they are largely overlapping with AEDs doses already used in humans for treating epilepsy/neuropathic pain. The main mechanism of antinociceptive/antihyperalgesic action of gabapentinoids in inflammatory pain models seems to be α₂δ-dependent suppression of voltage-gated calcium channels in primary sensory neurons that leads to reduced release of neurotransmitters in the spinal/medullar dorsal horn. The suppression of NMDA receptors via co-agonist binding site primarily at spinal sites, activation of various types of K⁺ channels at spinal and peripheral sites, and activation of noradrenergic and serotonergic descending pain modulatory pathways may also contribute. Inhibition of voltage-gated sodium channels along the pain pathway is probably the main mechanism of antinociceptive/antihyperalgesic effects of dibenzazepines. The recruitment of peripheral adrenergic and purinergic mechanisms and central GABAergic mechanisms may also contribute. When co-administered with classic/other alternative analgesics, AEDs exerted synergistic/additive interactions. Reviewed data could serve as a basis for clinical studies on the efficacy/safety of AEDs as analgesics/adjuvants in patients with inflammatory pain, and contribute to the improvement of the treatment of various inflammatory pain states.

Botulinum toxin type A and gabapentin attenuate postoperative pain and NK1 receptor internalization in rats

Treatment of postoperative pain remains a challenge in clinic. Botulinum toxin type A (BoNT/A) and gabapentin regulate the release of neurotransmitters from primary afferent neurons, but their effects of on postoperative pain are not clear. In the current study, using pain behavioral tests, Western blot analysis, and immunocytochemistry, we examined whether BoNT/A, alone or in combination with intrathecal gabapentin, inhibited pain hypersensitivity and attenuated the increase in neurokinin 1 (NK1) receptor internalization in dorsal horn neurons after plantar incision. Our data showed that pretreatment of rats with an intraplantar (2 U) 24 h before plantar incision or intrathecal (0.5 U) injection of BoNT/A 48 h before plantar incision induced a prolonged (3-5 days) decrease in pain scores and mechanical hypersensitivity, as compared to those observed with saline pretreatment. Both intraplantar and intrathecal BoNT/A pretreatment reduced synaptosomal-associated protein 25 levels in the ipsilateral lumbar dorsal root ganglia and spinal cord dorsal horn, and attenuated the increase in NK1 receptor internalization in dorsal horn neurons. Intrathecal administration of a sub-effective dose of gabapentin (50 μg) with BoNT/A (0.5 U) induced greater inhibition of pain hypersensitivity and NK1 receptor internalization than BoNT/A alone. These findings suggest that pretreatment with BoNT/A, alone or in combination with intrathecal gabapentin, may present a promising multimodal analgesia regimen for postoperative pain treatment.

Postoperative pain-from mechanisms to treatment

INTRODUCTION

Pain management after surgery continues to be suboptimal; there are several reasons including lack of translation of results from basic science studies and scientific clinical evidence into clinical praxis.

OBJECTIVES

This review presents and discusses basic science findings and scientific evidence generated within the last 2 decades in the field of acute postoperative pain.

METHODS

In the first part of the review, we give an overview about studies that have investigated the pathophysiology of postoperative pain by using rodent models of incisional pain up to July 2016. The second focus of the review lies on treatment recommendations based on guidelines and clinical evidence, eg, by using the fourth edition of the "Acute Pain Management: Scientific Evidence" of the Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine.

RESULTS

Preclinical studies in rodent models characterized responses of primary afferent nociceptors and dorsal horn neurons as one neural basis for pain behavior including resting pain, hyperalgesia, movement-evoked pain or anxiety- and depression-like behaviors after surgery. Furthermore, the role of certain receptors, mediators, and neurotransmitters involved in peripheral and central sensitization after incision were identified; many of these are very specific, relate to some modalities only, and are unique for incisional pain. Future treatment should focus on these targets to develop therapeutic agents that are effective for the treatment of postoperative pain as well as have few side effects. Furthermore, basic science findings translate well into results from clinical studies. Scientific evidence is able to point towards useful (and less useful) elements of multimodal analgesia able to reduce opioid consumption, improve pain management, and enhance recovery.

CONCLUSION

Understanding basic mechanisms of postoperative pain to identify effective treatment strategies may improve patients' outcome after surgery.

Comparative study of preoperative use of oral gabapentin, intravenous dexamethasone and their combination in gynaecological procedure

BACKGROUND

We studied the effects of oral gabapentin and intravenous (I.V.) dexamethasone given together or separately 1 h before the start of surgery on intraoperative hemodynamics Postoperative analgesia and postoperative nausea vomiting (PONV) in patients undergoing gynaecological procedure.

MATERIALS AND METHODS

Patients were randomly divided into three groups: Group 1 (gabapentin, n = 46) received 400 mg gabapentin, Group 2 (dexamethasone, n = 46) received 8 mg dexamethasone and Group 3 (gabapentin plus dexamethasone, n = 46) received both 400 mg gabapentin and 8 mg dexamethasone I.V. 1 h before the start of surgery. Standard induction and maintenance of anesthesia were accomplished. Visual analog scale for pain was recorded for 12 h. Side effects were noted.

RESULTS

Hemodynamics at various time interval (0, 5, 10, 15, 20, 25 and 30 min) of laryngeal mask airway insertion and PONV were found significantly lower in Group 3 than in Group 1 and Group 2 (P < 0.05). The average time to first postoperative analgesic requirement at (visual analogue score >3) was significantly longer in Group 3 (510.00 ± 61.64 min) than in Group 1 (352.83 ± 80.61 min) and in Group 2 (294.78 ± 60.76 min), (P < 0.05).

CONCLUSION

The present study concludes that the combination of oral Gabapentin and I.V. dexamethasone has significantly less hemodynamic changes, better postoperative analgesia and less incidence of PONV than individual administration of each drug.

Spontaneous Chronic Pain After Experimental Thoracotomy Revealed by Conditioned Place Preference: Morphine Differentiates Tactile Evoked Pain From Spontaneous Pain

Chronic pain after surgery limits social activity, interferes with work, and causes emotional suffering. A major component of such pain is reported as resting or spontaneous pain with no apparent external stimulus. Although experimental animal models can simulate the stimulus-evoked chronic pain that occurs after surgery, there have been no studies of spontaneous chronic pain in such models. Here the conditioned place preference (CPP) paradigm was used to reveal resting pain after experimental thoracotomy. Male Sprague Dawley rats received a thoracotomy with 1-hour rib retraction, resulting in evoked tactile hypersensitivity, previously shown to last for at least 9 weeks. Intraperitoneal injections of morphine (2.5 mg/kg) or gabapentin (40 mg/kg) gave equivalent 2- to 3-hour-long relief of tactile hypersensitivity when tested 12 to 14 days postoperatively. In separate experiments, single trial CPP was conducted 1 week before thoracotomy and then 12 days (gabapentin) or 14 days (morphine) after surgery, followed the next day by 1 conditioning session with morphine or gabapentin, both versus saline. The gabapentin-conditioned but not the morphine-conditioned rats showed a significant preference for the analgesia-paired chamber, despite the equivalent effect of the 2 agents in relieving tactile allodynia. These results show that experimental thoracotomy in rats causes spontaneous pain and that some analgesics, such as morphine, that reduce evoked pain do not also relieve resting pain, suggesting that pathophysiological mechanisms differ between these 2 aspects of long-term postoperative pain. Perspective: Spontaneous pain, a hallmark of chronic postoperative pain, is demonstrated here in a rat model of experimental postthoracotomy pain, further validating the use of this model for the development of analgesics to treat such symptoms. Although stimulus-evoked pain was sensitive to systemic morphine, spontaneous pain was not, suggesting different mechanistic underpinnings.

A membrane-delimited N-myristoylated CRMP2 peptide aptamer inhibits CaV2.2 trafficking and reverses inflammatory and postoperative pain behaviors

Targeting proteins within the N-type voltage-gated calcium channel (CaV2.2) complex has proven to be an effective strategy for developing novel pain therapeutics. We describe a novel peptide aptamer derived from the collapsin response mediator protein 2 (CRMP2), a CaV2.2-regulatory protein. Addition of a 14-carbon myristate group to the peptide (myr-tat-CBD3) tethered it to the membrane of primary sensory neurons near surface CaV2.2. Pull-down studies demonstrated that myr-tat-CBD3 peptide interfered with the CRMP2-CaV2.2 interaction. Quantitative confocal immunofluorescence revealed a pronounced reduction of CaV2.2 trafficking after myr-tat-CBD3 treatment and increased efficiency in disrupting CRMP2-CaV2.2 colocalization compared with peptide tat-CBD3. Consequently, myr-tat-CBD3 inhibited depolarization-induced calcium influx in sensory neurons. Voltage clamp electrophysiology experiments revealed a reduction of Ca, but not Na, currents in sensory neurons after myr-tat-CBD3 exposure. Current clamp electrophysiology experiments demonstrated a reduction in excitability of small-diameter dorsal root ganglion neurons after exposure to myr-tat-CBD3. Myr-tat-CBD3 was effective in significantly attenuating carrageenan-induced thermal hypersensitivity and reversing thermal hypersensitivity induced by a surgical incision of the plantar surface of the rat hind paw, a model of postoperative pain. These effects are compared with those of tat-CBD3-the nonmyristoylated tat-conjugated CRMP2 peptide as well as scrambled versions of CBD3 and CBD3-lacking control peptides. Our results demonstrate that the myristoyl tag enhances intracellular delivery and local concentration of the CRMP2 peptide aptamer near membrane-delimited calcium channels resulting in pronounced interference with the calcium channel complex, superior suppression of calcium influx, and better antinociceptive potential.

Intrathecal gabapentin to treat chronic intractable noncancer pain

BACKGROUND

Oral gabapentin is approved as an anticonvulsant medication and to treat postherpetic neuralgia. Its nonopioid properties and presumed spinal site of analgesic action made the study on intrathecal gabapentin attractive to establish the minimum effective dose for a later, pivotal trial.

METHODS

The authors examined the safety and efficacy of intrathecal gabapentin in a randomized, blinded, placebo-controlled, multicenter trial in a heterogeneous cohort of candidates with chronic pain for intrathecal drug therapy.

RESULTS

Patients (N = 170) were randomized to receive continuous intrathecal gabapentin (0 [placebo], 1, 6, or 30 mg/day) during 22 days of blinded treatment after implantation of a permanent drug delivery system. The highest dose, 30 mg/day, was selected to maintain a safety margin below the 100-mg/day dose that was explored in a phase 1 study. The authors found no statistically significant difference in the primary outcome measure, which was the numerical pain rating scale and response rate after 3 weeks, for any dose versus placebo. Physical functioning, quality of life, and emotional functioning also revealed no differences. Small, nonsignificant changes occurred in opioid medication use. The most frequent device-related adverse events were transient postimplant (lumbar puncture) headache, pain, and nausea. The most frequent gabapentin-related adverse events were nausea, somnolence, headache, dizziness, fatigue, and peripheral edema.

CONCLUSION

Twenty-two days of intrathecal gabapentin did not demonstrate statistically significant or clinically meaningful analgesic effects. The study sponsor has no current plans for further studies. Drug-related adverse events were similar to those for oral gabapentin. Most device-related adverse events resulted from the implant surgery or anesthesia.

Electroacupuncture at 2/100 hz activates antinociceptive spinal mechanisms different from those activated by electroacupuncture at 2 and 100 hz in responder rats

We examined the effects of intrathecal injection of desipramine and fluoxetine (selective inhibitors of norepinephrine and 5-HT uptake, resp.), thiorphan and neostigmine (inhibitors of enkephalinase and acetylcholinesterase, resp.), gabapentin (a GABA releaser), and vigabatrin (an inhibitor of GABA-transaminase) on the antinociception induced by 2 Hz, 100 Hz, or 2/100 Hz electroacupuncture (EA) applied bilaterally to the Zusanli (ST36) and Sanyinjiao (SP6) acupoints using the rat tail-flick test. We show that 2 Hz EA antinociception lasts longer after the administration of drugs that increase the spinal availability of norepinephrine, acetylcholine, or GABA; 100 Hz EA antinociception lasts longer after drug that increases the spinal availability of norepinephrine; 2/100 Hz EA antinociception lasts longer after drugs that increase the spinal availability of endogenous opioids or GABA. We conclude that the antinociceptive effect of 2/100 Hz EA is different from the synergistic effect of alternate stimulation at 2 and 100 Hz because the effect of the former is not changed by increasing the spinal availability of serotonin and lasts longer after the administration of vigabatrin. The combination of EA with drugs that increase the availability of spinal neurotransmitters involved in the modulation of nociceptive inputs may result in a synergistic antinociceptive effect in the rat tail-flick test.

The effect of intrathecal gabapentin on neuropathic pain is independent of the integrity of the dorsolateral funiculus in rats

AIM

This study evaluates the contribution of inhibitory pain pathways that descend to the spinal cord through the dorsolateral funiculus (DLF) on the effect of intrathecal gabapentin against spinal nerve ligation (SNL)-induced behavioral hypersensitivity to mechanical stimulation in rats.

MAIN METHOD

Rats were submitted to a sham or complete ligation of the right L5 and L6 spinal nerves and a sham or complete DLF lesion. Next, the changes induced by intrathecal administration of gabapentin on the paw withdrawal threshold of rats to mechanical stimulation were evaluated electronically.

KEY FINDINGS

Intrathecal gabapentin (200μg/5μl) that was injected 2 or 7days after surgery fully inhibited the SNL-induced behavioral hypersensitivity to mechanical stimulation in sham DLF-lesioned rats; gabapentin was effective against the SNL-induced behavioral hypersensitivity to mechanical stimulation also in DLF-lesioned rats.

SIGNIFICANCE

The effect of intrathecally administered gabapentin against SNL-induced behavioral hypersensitivity to mechanical stimulation in rats does not depend on the activation of nerve fibers that descend to the spinal cord via the DLF.

Gabapentin augments the antihyperalgesic effects of diclofenac sodium through spinal action in a rat postoperative pain model

BACKGROUND

Gabapentin and nonsteroidal antiinflammatory drugs (NSAIDs) attenuate postoperative pain and neuropathic pain in humans. The combination of gabapentin and NSAIDs is effective for postoperative pain and enhances functional recovery after surgery. Intrathecal administration of gabapentin or NSAIDs inhibits hyperalgesia in a rat postoperative pain model. However, there is no information on the effects of intrathecal administration of a combination of gabapentin and NSAIDs. We therefore investigated the effects of intrathecal administration of gabapentin and NSAIDs in a rat model of postoperative pain.

METHODS

Rats were prepared for intrathecal catheters under halothane anesthesia. Two days after catheterization, gabapentin (4, 40, or 400 μg per 20 μL of saline), diclofenac sodium, a nonselective cyclooxygenase inhibitor (2, 20, or 200 μg per 20 μL of 6% glucose), 20 μL saline, 20 μL 6% glucose, and a combination of gabapentin and diclofenac (40 μg gabapentin + 20 μg diclofenac and 4 μg gabapentin + 2 μg diclofenac per 20 μL 6% glucose) were injected intrathecally. We performed a hindpaw incision 30 minutes after injection. Each group consisted of 6 rats. The mechanical threshold was measured to evaluate secondary hyperalgesia using von Frey filaments before intrathecal catheterization and at 2 hours, and 1, 3, 5, and 7 days after paw incision.

RESULTS

Gabapentin 400 μg attenuated mechanical hyperalgesia for 7 days compared with the control group. Diclofenac 200 μg inhibited hyperalgesia for 5 days compared with the control group. The 40 μg gabapentin + 20 μg diclofenac group had a significantly reduced secondary hyperalgesic response in 2 hours and 1 day compared with 40 μg gabapentin and 20 μg diclofenac, respectively. The 4 μg gabapentin + 2 μg diclofenac group had a significantly reduced secondary hyperalgesic response in 2 hours and 1 day compared with 2 μg diclofenac. The withdrawal threshold on the contralateral paw did not change compared with the preincision threshold.

CONCLUSION

Intrathecal administration of gabapentin and diclofenac in combination reduced secondary hyperalgesia at doses having no antihyperalgesic effects when given individually. Our results suggest that gabapentin and diclofenac have an important role in postoperative pain reduction at the spinal level, and that gabapentin augments the antihyperalgesic effects of diclofenac through action in the spinal cord.

The combined analgesic effect of gabapentin and transdermal fentanyl patch on acute and chronic pain after maxillary cancer surgeries

Purpose of this study was to evaluate the combined analgesic effect of gabapentin and transdermal fentanyl patch, on acute and chronic pain after surgery for maxillary cancer.

STUDY DESIGN

The Study was double blind and prospective. 100 subjects belonging to ASA grade I and II, 30-50 years age group, scheduled for maxillary cancer surgery were randomized into two groups; treatment group (GT): to receive gabapentin, transdermal fentanyl patch or control group ©: two placebos. For acute postoperative pain (Visual Analogue Score) and analgesic requirements were assessed 2, 4, 8 hours and 7 days after surgery. Subjects were also assessed for chronic pain 2, 4, 6 months later.

RESULTS

Subjects in treatment group required lesser dose of analgesic, as compared to control group, in the post operative period. Visual Analogue Scores were also significantly lower in the treatment group throughout the post operative period. Occurrence of side effects was non significant between both groups. 2, 4 and 6 months after surgery, 40, 35 and 28 subjects respectively, out of total 45 subjects of the control group, reported chronic pain. In comparison, 25, 10 and 4 subjects out of 42 subjects in the treatment group reported chronic pain 2, 4, 6 months respectively after surgery. 15, 10 and 6 out of 45 of the control group required analgesics, whereas 2, 0 and 0 out of 42 in the treatment group, required analgesics respectively 2, 4 and 6 months after surgery

CONCLUSION

Acute and chronic pain after maxillary cancer surgery is significantly reduced by multimodal analgesia.

The synergistic interaction between rilmenidine and paracetamol in the writhing test in mice

The aim of the study was to ascertain antinociceptive effects of rilmenidine, a second-generation imidazoline-alpha-2-adrenoreceptor agonist, and to see whether rilmenidine was able to increase the analgesic effects of paracetamol in the writhing test in mice. An acetic acid (0.7%) solution was injected into the peritoneal cavity and the number of writhes was counted. The influence on locomotor performance was tested using the rotarod test. Rilmenidine, paracetamol, and rilmenidine-paracetamol fixed-ratio combinations produced dose-dependent antinociceptive effects. ED(50) values were estimated for the individual drugs and an isobologram was constructed. The derived theoretical additive ED(50) value for the rilmenidine-paracetamol combination was 109.23 +/- 35.05 mg/kg. This value was significantly greater than the observed ED(50) value which was 56.35 +/- 20.86 mg/kg, indicating a synergistic interaction. Rilmenidine did not impair motor coordination, as measured by the rotarod test, at antinociceptive and higher doses.

Synergy between a NR2B receptor antagonist DHQ and 3-methyl-gabapentin in mice with neuropathic pain

The synergistic effect of a selective NR2B NMDA receptor antagonist, (-)-(R)-6-{2-[4-(3-Fluorophenyl)-4-hydroxy-1-piperidinyl]-1-hydroxyethyl-3,4-dihydro-2(1H)-quinolinone (DHQ), and a alpha 2 delta ligand, 3-methyl-gabapentin (3M-GBP), was investigated in the mouse partial sciatic nerve model. The interaction was observed after administration of DHQ and 3M-GBP combination at fixed dose ratios of 1:10 and 1:30 and the dose-response curves shifted approximately 13- and 17-fold leftward, respectively, from the theoretical additive values. However, a fixed dose ratio of 1:50 resulted only in an additive effect. These results indicate the synergistic interaction between DHQ and 3M-GBP in this animal model of neuropathic pain.

Gabapentin reduces cardiovascular responses to laryngoscopy and tracheal intubation

BACKGROUND AND OBJECTIVE

We have compared the effects of gabapentin on arterial pressure and heart rate at induction of anaesthesia and tracheal intubation in a randomized double-blind study.

METHODS

Ninety normotensive patients (ASA I) undergoing elective surgery were divided into three groups of 30 patients each. Patients received oral placebo (Group I), 400 mg of gabapentin (Group II) or 800 mg of gabapentin (Group III) 1 h prior to surgery in the operating theatre. After induction of anaesthesia heart rate and mean arterial pressure were recorded at baseline 1, 3, 5, 10 and 15 min after intubation.

RESULTS

Patients receiving placebo and 400 mg gabapentin showed a significant increase in blood pressure and heart rate associated with tracheal intubation compared to baseline levels and Group III. There was significant decrease in heart rate and arterial pressure in Group III after intubation 1, 3, 5 and 10 min (P < 0.001, P < 0.001, P < 0.05 and P < 0.05, respectively) compared to Groups I and II.

CONCLUSION

Given 1 h before operation gabapentin 800 mg blunted the arterial pressure and heart rate increase in first 10 min due to endotracheal intubation. Oral administration of gabapentin 800 mg before induction of anaesthesia is a simple and practical method for attenuating pressor response to laryngoscopy and tracheal intubation after standard elective induction.

The preoperative use of gabapentin, dexamethasone, and their combination in varicocele surgery: a randomized controlled trial

BACKGROUND

We investigated the effects of gabapentin and dexamethasone given together or separately 1 h before the start of surgery on laryngoscopy, tracheal intubation, intraoperative hemodynamics, opioid consumption, and postoperative pain in patients undergoing varicocele operations.

METHODS

Patients were randomly divided into four double-blind groups: group C (control, n = 20) received placebo, group G (gabapentin, n = 20) received 800 mg gabapentin, group D (dexamethasone, n = 20) received 8 mg dexamethasone, group GD (gabapentin plus dexamethasone) received both 800 mg gabapentin and 8 mg dexamethasone IV 1 h before the start of surgery. Standard induction and maintenance of anesthesia were accomplished and continued by propofol and remifentanil infusion. Heart rate and arterial blood pressure were recorded before induction and after intubation. Intraoperative total remifentanil consumption was recorded. Hemodynamic variables and visual analog scale were recorded for 24 h. Side effects were noted.

RESULTS

Hemodynamics at 1, 3, 5, and 10 min after tracheal intubation, total remifentanil consumption during surgery, postoperative visual analog scale scores at 30 min, 1, 2, 4, 6, and 12 h, and postoperative nausea and vomiting were found to be significantly lower in group GD than in group G and group D (P < 0.05 for both), and substantially lower when compared with group C (P < 0.001). All values in group C were also higher than in groups G and D (P < 0.05).

CONCLUSION

Gabapentin and dexamethasone administered together an hour before varicocele surgery results in less laryngeal and tracheal intubation response, improves postoperative analgesia, and prevents postoperative nausea and vomiting better than individual administration of each drug.

Pregabalin, S-(+)-3-isobutylgaba, activates the descending noradrenergic system to alleviate neuropathic pain in the mouse partial sciatic nerve ligation model

We have previously demonstrated that gabapentin supraspinally activates the descending noradrenergic system to alleviate neuropathic pain. In this study, we investigated whether pregabalin, an antiepileptic and analgesic drug that is also designed as a structural analogue of gamma-aminobutyric acid (GABA), exhibits supraspinal analgesic effects similar to those of gabapentin involving the descending noradrenergic system. Both systemically (intraperitoneally; i.p.) and locally (intracerebroventricularly or intrathecally; i.c.v. or i.t.) injected pregabalin reduced thermal and mechanical hypersensitivity in a murine chronic pain model that was prepared by partial ligation of the sciatic nerve (the Seltzer model), suggesting that pregabalin acts at both supraspinal and spinal loci. The supraspinal analgesic action of pregabalin was observed only after peripheral nerve injury, and pregabalin (i.p. and i.c.v.) did not affect acute thermal and mechanical nociception. Depletion of spinal noradrenaline (NA) or pharmacological blockade of spinal alpha(2)-adrenoceptors with yohimbine (i.p. or i.t.), but not alpha(1)-adrenoceptors with prazosin (i.p.), reduced the analgesic effects of pregabalin (i.p. or i.c.v.) on thermal and mechanical hypersensitivity. Moreover, i.c.v.-administered pregabalin dose-dependently increased the spinal 4-hydroxy-3-methoxyphenylglycol (MHPG) content and the MHPG/NA ratio only in mice with neuropathic pain, whereas the concentrations of NA, serotonin, 5-hydroxyindoleacetic acid and dopamine were unchanged, demonstrating that supraspinal pregabalin accelerated the spinal turnover of NA. Together, these results indicate that pregabalin supraspinally activates the descending noradrenergic pain inhibitory system coupled with spinal alpha(2)-adrenoceptors to ameliorate neuropathic pain.

Postoperative pain--clinical implications of basic research

Postoperative incisional pain is a unique and common form of acute pain. Although ample evidence indicates that an efficeous postoperative pain treatment reduces patient morbidity and patient outcome, recent studies demonstrate that about 50-70% of patients experience moderate to severe pain after surgery indicating that postoperative pain remains poorly treated. Perhaps important reasons for this quandary are distinct mechanisms of incisional nociception compared to other pain conditions limiting our regimen to drugs designed for other clinical pain problems. Another reason might be the lack of an in depth knowledge about the pathophysiology and neuropharmacology of postoperative pain. Basic research offers important insights in the mechanisms of postsurgical incisional pain and the translation of experimental results into clinical practice will have important implications on the improvement of new multimodal treatment regimens based postoperative pain mechanisms. In the present review, recent developments in experimental postsurgical incisional pain research will be described and their possible relevance for clinical practice discussed.

Neurochemical evidence that supraspinally administered gabapentin activates the descending noradrenergic system after peripheral nerve injury

We have previously demonstrated that gabapentin supraspinally activates the descending noradrenergic system to produce analgesic effects after peripheral nerve injury. To further establish the neurochemical basis for its supraspinally mediated analgesic action, concentrations of spinal noradrenaline, 4-hydroxy-3-methoxyphenylglycol (MHPG), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and dopamine were measured using high-performance liquid chromatography in a murine neuropathic pain model that was prepared by partial ligation of the sciatic nerve (the Seltzer model). Intracerebroventricularly (i.c.v.) administered gabapentin (100 and 300 microg) increased the spinal MHPG concentration and the MHPG/noradrenaline ratio and alleviated mechanical hypersensitivity, whereas the concentrations of noradrenaline, 5-HT, 5-HIAA and dopamine were unchanged. By contrast, i.c.v. gabapentin neither affected the spinal MHPG concentration and MHPG/noradrenaline ratio nor exhibited analgesic effects in animals subjected to a sham operation. In addition, spinal monoamine levels in ligated animals were not changed after intrathecal administration of gabapentin which however generated analgesic effects. Thus, the supraspinally mediated analgesic effects of gabapentin are correlated with an increase in spinal noradrenaline turnover.

Mechanisms of the antinociceptive action of gabapentin

Gabapentin, a gamma-aminobutyric acid (GABA) analogue anticonvulsant, is also an effective analgesic agent in neuropathic and inflammatory, but not acute, pain systemically and intrathecally. Other clinical indications such as anxiety, bipolar disorder, and hot flashes have also been proposed. Since gabapentin was developed, several hypotheses had been proposed for its action mechanisms. They include selectively activating the heterodimeric GABA(B) receptors consisting of GABA(B1a) and GABA(B2) subunits, selectively enhancing the NMDA current at GABAergic interneurons, or blocking AMPA-receptor-mediated transmission in the spinal cord, binding to the L-alpha-amino acid transporter, activating ATP-sensitive K(+) channels, activating hyperpolarization-activated cation channels, and modulating Ca(2+) current by selectively binding to the specific binding site of [(3)H]gabapentin, the alpha(2)delta subunit of voltage-dependent Ca(2+) channels. Different mechanisms might be involved in different therapeutic actions of gabapentin. In this review, we summarized the recent progress in the findings proposed for the antinociceptive action mechanisms of gabapentin and suggest that the alpha(2)delta subunit of spinal N-type Ca(2+) channels is very likely the analgesic action target of gabapentin.

Gabapentin provides effective postoperative analgesia whether administered pre-emptively or post-incision

PURPOSE

We investigated the effects of pre-incision and post-incision administration of gabapentin on postoperative pain and fentanyl consumption associated with open donor nephrectomy.

METHODS

Sixty ASA I subjects were randomly allocated into three groups to receive gabapentin 600 mg two hours before surgery and placebo after surgical incision (pre-incision group), placebo two hours before surgery and gabapentin 600 mg after surgical incision (post-incision group), or placebo two hours before surgery and after surgical incision (placebo group). After surgery, pain was assessed using a visual analogue scale (VAS), (1-10 cm) at time points 0, 6, 12, 18, and 24 hr. Subjects received patient-controlled-analgesia (fentanyl 1.0 microg x kg(-1) subject activated dose). Total fentanyl consumption in each group was recorded.

RESULTS

Subjects of pre-incision and post-incision groups had lower VAS scores at all time points (3.1 +/- 1.8, 2.9 +/- 1.3, 2.8 +/- 1.3, 2.5 +/- 0.9, 2.5 +/- 1.5 and 3.6 +/- 1.1, 3.0 +/- 1.2, 3.2 +/- 1.1, 2.9 +/- 1.0, 2.6 +/- 2.2) compared to placebo group (6.6 +/- 1.3, 5.0 +/- 1.0, 4.4 +/- 0.7, 4.2 +/- 0.8, 3.9 +/- 1.0). They also used less fentanyl (563.3 microg +/- 252.8 and 624.0 microg +/- 210.5 respectively) compared to placebo (924.7 microg +/- 417.5), (P < 0.05). No difference in total fentanyl consumption and pain scores at any time points were observed between pre- and post-incision groups.

CONCLUSION

Pre-incision administration of 600 mg gabapentin has no added benefit over post-incision administration in terms of pain scores and fentanyl consumption in subjects undergoing open donor nephrectomy.

Multimodal analgesia with gabapentin and local anesthetics prevents acute and chronic pain after breast surgery for cancer

We evaluated the effect of multimodal analgesia on acute and chronic pain after breast surgery for cancer. Fifty patients scheduled for breast cancer surgery were blindly randomized to receive gabapentin, eutectic mixture of local anesthetics cream, and ropivacaine in the wound or three placebos. Pain (visual analog scale) and analgesics were recorded in the postanesthesia care unit (PACU) 3, 6, and 9 h and 8 days after surgery. Three and 6 mo later, patients were assessed for chronic pain. The treatment group consumed less paracetamol in the PACU (469 versus 991 mg; P < 0.002) and less Lonalgal (1.0 versus 4.4 tablets; P = 0.003) than the controls, exhibited lower visual analog scale scores at rest in the PACU (P = 0.001) and on postoperative Days 1, 3, and 5 (P = 0.040, P = 0.015, and P = 0.045, respectively), and after movement in the PACU (P = 0.001) and on postoperative Days 2, 4, and 8 (P = 0.028, P = 0.007, and P = 0.032, respectively). Three and 6 mo after surgery, 18 of 22 (82%) and 12 of 21 (57%) of the controls reported chronic pain versus 10 of 22 (45%) and 6 of 20 (30%) in the treatment group (P = 0.028 and P = 0.424, respectively); 5 of 22 and 4 of 21 of the controls required analgesics versus 0 of 22 and 0 of 20 of those treated (P = 0.048 and P = 0.107, respectively). Multimodal analgesia reduced acute and chronic pain after breast surgery for cancer.

Role of descending noradrenergic system and spinal alpha2-adrenergic receptors in the effects of gabapentin on thermal and mechanical nociception after partial nerve injury in the mouse

1. To gain further insight into the mechanisms underlying the antihyperalgesic and antiallodynic actions of gabapentin, a chronic pain model was prepared by partially ligating the sciatic nerve in mice. The mice then received systemic or local injections of gabapentin combined with either central noradrenaline (NA) depletion by 6-hydroxydopamine (6-OHDA) or alpha-adrenergic receptor blockade. 2. Intraperitoneally (i.p.) administered gabapentin produced antihyperalgesic and antiallodynic effects that were manifested by elevation of the withdrawal threshold to a thermal (plantar test) or mechanical (von Frey test) stimulus, respectively. 3. Similar effects were obtained in both the plantar and von Frey tests when gabapentin was injected intracerebroventricularly (i.c.v.) or intrathecally (i.t.), suggesting that it acts at both supraspinal and spinal loci. This novel supraspinal analgesic action of gabapentin was only obtained in ligated neuropathic mice, and gabapentin (i.p. and i.c.v.) did not affect acute thermal and mechanical nociception. 4. In mice in which central NA levels were depleted by 6-OHDA, the antihyperalgesic and antiallodynic effects of i.p. and i.c.v. gabapentin were strongly suppressed. 5. The antihyperalgesic and antiallodynic effects of systemic gabapentin were reduced by both systemic and i.t. administration of yohimbine, an alpha2-adrenergic receptor antagonist. By contrast, prazosin (i.p. or i.t.), an alpha1-adrenergic receptor antagonist, did not alter the effects of gabapentin. 6. It was concluded that the antihyperalgesic and antiallodynic effects of gabapentin are mediated substantially by the descending noradrenergic system, resulting in the activation of spinal alpha2-adrenergic receptors.

Mechanisms of incisional pain

Postoperative, incisional pain is a unique but common form of acute pain. Because effective postoperative analgesia reduces morbidity following surgery, new treatments continue to be sought. It is through the development of investigational models and studies of the mechanisms that perioperative medicine can be advanced. This article reviews studies on a rat plantar hindpaw model for postoperative pain and proposes mechanisms for enhanced excitability of sensory neurons caused by incisions.

Pharmacokinetic-pharmacodynamic correlations and biomarkers in the development of COX-2 inhibitors

The mechanism by which COX inhibitors exert their analgesic effect is well established. However, data show no direct correlation between drug concentrations in plasma and the analgesic or adverse effects in chronic inflammatory conditions. This represents a major problem in the development of COX inhibitors, since it is difficult to predict the appropriate dosing regimen for the treatment of chronic inflammatory pain, based upon information from pre-clinical studies and eventually early clinical studies. The factors that determine response in inflammatory pain must be understood in order to make predictions about the time course of the analgesic effect. In this review the determinants of drug response and their variability will be discussed: physicochemical properties, pharmacokinetics (PK), pathophysiology and disease progression. From a mechanistic point of view, endogenous mediators of inflammation might be used as a biomarker for the analgesic effect and safety assessment. Such a biomarker can be an intermediate step between drug exposure and response. In addition, its concentration-effect relationship could be characterized by pharmacokinetic-pharmacodynamic (PK/PD) modelling. Indeed, recent investigations have shown that COX-2 inhibition, as determined by modelling of prostaglandin E2 (PGE2) levels in the whole blood assay in vitro can be used as a marker to predict drug effects (analgesia) in humans. A model-derived parameter, IC80, (total and unbound) was found to correlate directly with the analgesic plasma concentration of different COX inhibitors varying in enzyme selectivity. These findings indicate that PGE2 and thromboxane B2 inhibition can be used to predict and select efficacious doses in humans.

Antinociceptive interactions between intrathecal gabapentin and MK801 or NBQX in rat formalin test

Antagonists for spinal N-methyl-D-aspartate (NMDA) and amino-hydroxy-methtyl-isoxazolepropionate (AMPA) receptors are effective in attenuating acute nociception or injury-induced hyperalgesia. The antinociception of spinal gabapentin is developed in injury-induced hyperalgesia without affecting acute nociception. The authors evaluated the effects of intrathecal gabapentin, NMDA antagonist (MK801) and AMPA antagonist (NBQX) in the formalin test which shows injury-induced hyperalgesia as well as acute pain. We further assessed the interactions between gabapentin and either MK801 or NBQX. Male Sprague-Dawley rats were implanted with intrathecal catheters. To evoke pain, 50 microL of 5% formalin solution was injected into the hindpaw. The interaction was investigated by a fixed dose analysis or an isobolographic analysis. MK801 and NBQX suppressed flinching responses during phase 1 of the formalin test, while gabapentin had little effect on phase 1. All three agents decreased the phase 2 flinching response. A fixed dose analysis in phase 1 showed that gabapentin potentiated the antinociceptive effect of MK801 and NBQX. Isobolographic analysis in phase 2 revealed a synergistic interaction after coadministration of gabapentin-MK801 or gabapentin-NBQX. Correspondingly, spinal gabapentin with NMDA or AMPA antagonist may be useful in managing acute pain and injury-induced hyperalgesia.

Polyanalgesic Consensus Conference 2003: an update on the management of pain by intraspinal drug delivery-- report of an expert panel

Intraspinal drug infusion using fully implantable pump and catheter systems is a safe and effective therapy for selected patients with chronic pain. The options for this approach are increasing, as drugs that are commercially available for systemic administration are adapted to this use and other drugs that are in development specifically for intraspinal administration become available. In 2000 a Polyanalgesic Consensus Conference was organized to evaluate the existing literature and develop guidelines for drug selection. The major outcome of this effort, an algorithm for drug selection, was based on the best available evidence at the time. Rapid changes have occurred in the science and practice of intraspinal infusion and a Polyanalgesic Consensus Conference 2003 was organized to pursue the following goals: 1) to review the literature on intraspinal drug infusion since 1999, 2) to revise the 2000 drug-selection algorithm, 3) to develop guidelines for optimizing drug dosage and concentration, 4) to create a process for documenting minimum evidence supporting the use of a drug for intraspinal infusion, and 5) to clarify issues pertaining to compounding of drugs. Based on the best available evidence and expert opinion, consensus recommendations were developed in all these areas. The panel's conclusions may provide a foundation for clinical practice and a rational basis for new research.

The Analgesic Effects of Gabapentin After Total Abdominal Hysterectomy

We investigated, in a randomized, placebo-controlled, double-blind study, the efficacy and safety of gabapentin on pain after abdominal hysterectomy and on tramadol consumption in patients. The 50 patients were randomized to receive either oral placebo or gabapentin 1200 mg 1 h before surgery. Anesthesia was induced with propofol and maintained with sevoflurane in 50% N(2)O/O(2) with a fresh gas flow of 2 L/min (50% N(2)O in O(2)) and fentanyl (2 microg/kg). All patients received patient-controlled analgesia with tramadol with a 50 mg initial loading dose, 20 mg incremental dose, 10-min lockout interval, and 4-h limit of 300 mg. The incremental dose was increased to 30 mg if analgesia was inadequate after 1 h. Patients were studied at 4, 8, 12, 16, 20, and 24 h for visual analog (VAS) pain scores, heart rate, peripheral oxygen saturation, mean arterial blood pressure, respiratory rate, sedation, and tramadol consumption. The VAS scores in the sitting and supine position at 1, 4, 8, 12, 16, and 20 h were significantly lower in the gabapentin group when compared with the placebo group up to 20 h after surgery. The tramadol consumption at 12, 16, 20, and 24 h and total tramadol consumption were significantly less in the gabapentin group when compared with placebo group. Sedation scores were similar at all the measured times. There were no differences between groups in adverse effects. Preoperative oral gabapentin decreased pain scores and postoperative tramadol consumption in patients after abdominal hysterectomy.

IMPLICATIONS

This randomized, controlled trial examined the effects of preoperative oral gabapentin 1200 mg on postoperative pain and tramadol consumptions. We conclude that preoperative oral gabapentin is effective in reducing postoperative pain scores and tramadol consumption in patients after abdominal hysterectomy.

Characteristic of Interactions Between Intrathecal Gabapentin and Either Clonidine or Neostigmine in the Formalin Test

Intrathecal gabapentin is effective for phase 2 of the formalin response but not for acute pain. Unlike gabapentin, intrathecal clonidine and neostigmine attenuate both acute pain and phase 2 of the formalin response. We evaluated gabapentin's interactions with either clonidine or neostigmine in the formalin test. Male Sprague-Dawley rats were used. For the formalin test, 50 microL of 5% formalin solution was injected into the hindpaw. The interaction of drugs was investigated by a fixed-dose analysis or an isobolographic analysis. Intrathecal gabapentin produced a suppression of the phase 2 flinching response, but not the phase 1 response, in the formalin test. Intrathecal clonidine and neostigmine resulted in a reduction of the pain behavior in both phases. A fixed-dose analysis in phase 1 showed that gabapentin potentiated the antinociceptive effect of clonidine and neostigmine. An isobolographic analysis in phase 2 revealed a synergistic interaction after intrathecal administration of gabapentin-clonidine or gabapentin-neostigmine mixture. We conclude that the combination of gabapentin with either clonidine or neostigmine at the level of the spinal cord could play a major role not only in acute pain, but also in phase 2 of the formalin response.

IMPLICATIONS

We determined the pharmacological properties of gabapentin combined with either clonidine or neostigmine in the formalin test. Spinal gabapentin reinforced the effects of clonidine and neostigmine in the formalin test. The hitherto unreported action of gabapentin on acute nociceptive stimulus could be of considerable significance.

Does gabapentin act as an agonist at native GABAB receptors?

Gabapentin, a novel anticonvulsant and analgesic, is a gamma-aminobutyric acid (GABA) analogue but was shown initially to have little affinity at GABA(A) or GABA(B) receptors. It was recently reported to be a selective agonist at GABA(B) receptors containing GABA(B1a)-GABA(B2) heterodimers, although several subsequent studies disproved that conclusion. In the present study, we examined whether gabapentin is an agonist at native GABA(B) receptors using a rat model of postoperative pain in vivo and periaqueductal gray (PAG) slices in vitro; PAG contains GABA(B) receptors, and their activation results in antinociception. An intrathecal injection of gabapentin or baclofen, a GABA(B) receptor agonist, induced antiallodynia in this postoperative pain model. Intrathecal injection of GABA(B) receptor antagonists CGP 35348 and CGP 55845 antagonized baclofen- but not gabapentin-induced antiallodynia. In ventrolateral PAG neurons, baclofen activated G-protein-coupled inwardly rectifying K(+) (GIRK) channels in a manner blocked by CGP 35348 or CGP 55845. However, gabapentin displayed no effect on the membrane current. In neurons unaffected by gabapentin, baclofen activated GIRK channels through GABA(B) receptors. It is concluded that gabapentin is not an agonist at GABA(B) receptors that are functional in baclofen-induced antiallodynia in the postoperative pain model in vivo and in GIRK channel activation in ventrolateral PAG neurons in vitro.